In electrophotographic reproduction apparatus and printers, an electrostatic latent image is formed on a photoconducting imaging member by first uniformly charging the imaging member and then image-wise exposing the imaging member using various devices such as a scanned laser, LED array, optical flash, or other suitable, known methods. The electrostatic latent image is then developed into a visible image by bringing the imaging member into close proximity with a developer that includes toner particles. In a 2-component developer, toner particles are mixed with larger, magnetic particles called carrier particles. The toner and carrier particles often contain charge agents that enable the toner particles to become triboelectrically charged by contact with the carrier particles. The developer is contained in a development station that typically includes a roller with a magnetic core, a sump that contains a quantity of developer, a device for determining the concentration of toner in the developer, and a mechanism for replenishing the toner when the toner concentration drops below a certain level. The carrier particles transport the toner into contact with the imaging member bearing the electrostatic latent image. The development station is suitably biased and the toner particles suitably charged so that the proper amount of toner particles is deposited in either the charged or discharged regions of the imaging member.
After the electrostatic latent image on the imaging member has been developed, the toned image is generally transferred to a receiver such as paper or transparency stock. This is generally accomplished by applying an electric field in such a manner to urge the toner from the imaging member to the receiver. In some instances, it is preferable to first transfer the toned image from the imaging member to an intermediate member and then from the intermediate member to the receiver. Again, this is most commonly accomplished by applying an electric field to urge the toned image towards the appropriate member.
The electrophotographic imaging process described above may be used to produce mono-color, typically black, or multi-color images. In so-called full-color or process-color imaging, toner pigmented with the subtractive primary colors, cyan, magenta, and yellow, are used along with black toner. Cyan, magenta, yellow, and black developed toner images are created separately by the above described process and transferred in register to the receiver. This process is typically used for pictorial imaging. A range or gamut of colors is produced by the varying amounts of the subtractive primary colored toners plus black in the image. Alternatively, it is sometimes desirable to employ a spot color or custom color toner in a single developer station to create a single colored image. Corporate logos and the like are such applications. Custom color toner may be produced by incorporating a custom color pigment into the toner during the toner manufacturing process. A disadvantage of producing a custom color toner in this way is that the amount of custom color toner needed for a given application may be less than the amount that is cost effective to manufacture in a production run. An alternative method of producing a custom color toner, which avoids the above mentioned disadvantage, is to create the custom color toner by blending together appropriate amounts of component toners pigmented during manufacture with the subtractive primary colored pigments, cyan, magenta, and yellow. If the desired custom color is not within the gamut of the cyan, magenta, and yellow component toners, additional colored component toners may be used in the blended custom color toner. This method is analogous to the mixing of component color paints to produce a custom color paint. However, this alternative method of producing a custom color toner also has a disadvantage, which is described below.
The rate at which toner is developed, from the development station, onto the electrostatic latent image is dependent on several parameters, including the toner charge, specifically the toner charge normalized to the mass of the toner particle and designated as charge-to-mass (q/m). As described above, the toner is charged by triboelectric interaction with the magnetic carrier particles. The toner charge is determined, in part, by the choice of charge agents incorporated into the toner. However, toner q/m may also depend on the toner particle size. Since the toner is charged through a triboelectric process, the more surface area available, the higher the value of q/m can be. Since smaller particles have higher surface area for a given mass than larger particles, q/m tends to increase as the size of the toner decreases. In addition, the different pigments used in the component toners also tend to have different triboelectric properties. This results in different component color toners potentially having different q/m ratios if mixed with the same carrier to form a developer. If the q/m of the component toners blended to make a custom color toner are significantly different, the components of the blended toner will develop the electrostatic latent image at different rates, thereby causing the color of the blended toner to vary with use.